The present invention relates to a heat pump system having an indoor unit and an outdoor unit.
A conventional heat pump of this type is as shown in FIG. 1.
In FIG. 1, reference numeral 1 designates a compressor; 2, a four-way valve; 3, an outdoor heat exchanger; 4, a distributor; 5, an expansion valve; 6, a connecting pipe; 7, an indoor heat exchanger; 8, a connecting pipe; and 9, an accumulator. At least the indoor heat exchanger 7 is included in the indoor unit, and the members which are not included in the indoor unit are included in the outdoor unit.
The cooling cycle is as follows: A high-temperature, high-pressure refrigerant discharged from the compressor 1 during the room cooling operation and a lubricating refrigerator oil mixed with the refrigerant flow through the four-way valve 2 to the outdoor heat exchanger 3 where they are changed into a high-temperature, high-pressure liquid refrigerant by heat-exchange. The liquid refrigerant is delivered via the distributor 4 through the expansion valve 5 where its pressure is decreased. The liquid refrigerant thus treated is sent through the pipe 6 to the indoor heat exchanger 7 where it is evaporated. The vapor thus formed flows through the pipe 8, the four-way valve 2 and the accumulator 9 to the compressor 1.
This conventional heat pump suffers from a drawback in the case where the connecting pipes 6 and 8 are long. Specifically, during the continuous operation of the compressor 1, the refrigerator oil mixed with the refrigerant discharged from the compressor 1 is also discharged continuously, and it takes a relatively long time until the refrigerator oil thus discharged returns to the compressor 1. Accordingly, the amount of refrigerator oil in the compressor 1 tends to decrease, as a result of which the compressor is not sufficiently lubricated and the sliding parts may seize. This difficulty may arise also in the room heating mode. Moreover, when the system is operating in a capacity control mode or low load mode, the amount of circulation of the refrigerant is reduced, as is the velocity of the refrigerant flowing in the pipe, as a result of which the amount of refrigerator oil returned to the compressor is decreased. Thus, as in the above-described case, lubrication of the compressor becomes insufficient.